Investigating the thermal response of Sarasota Bay dolphins to changing environmental temperatures
By “Team Thermal”: Ann Pabst, PhD, Bill. McLellan, Andrew Westgate, PhD, Erin Meagher, MSc, Michelle Barbieri, MSc, and Ari Friedlaender, MSc, University of North Carolina, Wilmington, and Duke University

The goal of our work with the Sarasota Dolphin Research Program is to better understand reproductive and whole-body temperature regulation (thermoregulation) in bottlenose dolphins. The long-term, health-monitoring program for Sarasota Bay dolphins offers us a unique opportunity to study thermoregulation in wild cetaceans. This year, we carried out the last of our investigations aimed at understanding how Sarasota Bay dolphins thermally adapt to seasonal changes in environmental temperatures. Their year-round residency exposes these dolphins to water temperatures that can drop below 10oC (50oF) in the winter and exceed 31oC (87.8oF) in the summer.

Bottlenose dolphins in Sarasota Bay may invoke a suite of physiological modifications to cope with their changing thermal environment. We investigated thermal function in dolphins using multiple measurement techniques, which included skin surface temperatures and heat flux values, measured at multiple positions on the dolphin’s body. Heat flux is the rate of energy transfer per unit area and measured in Watts/m2. Deep core temperatures, measured with a specialized colonic probe, and blubber thicknesses, measured using ultrasound, were also recorded. A dorsal fin Trac Pac was deployed on a subset of dolphins, which recorded skin surface temperatures and heat flux values, as well as velocity and time-depth records. These Trac Pacs were attached to the fin’s surface, using suction cups, and deployed for periods lasting up to 8 hours. Infrared thermal imaging (a form of digital photography) was used to measure skin surface temperatures of wild dolphins while they are free-swimming.

Our research team has collected this suite of physiological data on Sarasota dolphins during multiple health-monitoring studies, and this year, much of our thermal research has reached completion. For example, Michelle Barbieri completed her Master’s thesis research on surface temperatures of free-swimming dolphins. Her work demonstrated that dolphins maintain their surface temperature within about 1oC (33.8oF) of water temperature across all seasons. Changes in integumentary and vascular insulation likely account for the stability of this temperature differential, and, thus, the protection of core temperature across a large annual range in environmental temperature.

This year also represented the culmination of six years of thermal tracking work. We deployed our first Trac Pac thermal data logger in the summer of 1999 and our last during the summer of 2005. We have been fortunate enough to collect about 130 hours of data from 55 individual deployments in both winter and summer seasons. This effort represents one of the most comprehensive thermal data sets yet collected from free-ranging dolphins and we are looking forward to spending the next year analyzing and interpreting these results. We hope to be able to address a number of important thermal questions including (1) how do bottlenose dolphins respond thermally to seasonal changes in water temperatures that can vary by as much as 22 °C (71.6oF), (2) what are the “typical” thermal characteristics of free-ranging bottlenose dolphins and what sort of variability do we see across ages and sexes, and (3) do bottlenose dolphins respond thermally to changes in activity and what is the relationship between diving and heat loss? Andrew Westgate will begin a Post-Doctoral Fellowship with SDRP starting January 2006 and is looking forward to devoting considerable time to this unique data set.

Erin Meagher collected the final heat flux measurements for her PhD research this year as well. Cetaceans use their appendages (dorsal fin, pectoral flippers and flukes) to either conserve or dissipate body heat, thus, these body sites are considered thermal windows. Erin’s study re-examines the roles of the thermal windows and other body surfaces in regulating the body temperature of dolphins. Thus, we are mapping heat flux patterns over multiple body surfaces, including the appendages, tailstock and lateral body wall, in wild bottlenose dolphins. Assessing heat flux at multiple body sites simultaneously will elucidate whether dolphins prioritize one body surface or thermal window over another when dissipating excess body heat. In January 2005, experiments were conducted on six wild bottlenose dolphins (3 males, 3 females). These data were added to those collected in February 2003 and 2004 and June 2002, 2003 and 2004 for a total of 57 bottlenose dolphins sampled in winter and summer. Preliminary results suggest that mean heat flux from all body sites pooled in the winter was generally, although not significantly, higher than mean heat flux in the summer. Thus, the initial hypothesis that heat flux across a dolphin’s body surface would decrease in the winter in response to increased blubber thickness and decreased ambient water temperatures was not supported. These winter increases in heat flux occurred despite significantly thicker blubber layers at these sites in the winter. These results suggest that bottlenose dolphins resident to Sarasota Bay may be using alternative mechanisms to dissipate excess body heat in the summer, such as respiratory evaporative heat loss or spending more time in cooler microclimates. These alternative mechanisms are currently under investigation.

Our team has also completed a 10 year study on the ontogeny of the dolphin reproductive countercurrent heat exchanger. Male bottlenose dolphins possess a countercurrent heat exchanger (CCHE) that functions to regulate the temperature of their intra-abdominal testes, and we investigated the development of CCHE function by measuring deep body temperatures of wild Sarasota dolphins. During 14 field sessions (June 1993-February 2005), we collected deep body temperatures of 49 known-age males. Nineteen dolphins were sampled multiple times, over a span of 2-10 years. The CCHE flanks a region of the colon and in captive dolphins colonic temperatures measured within this region are cooler than those measured either cranially or caudally. Thus, we used a specially-constructed probe, housing a linear array of thermocouples, to measure colonic temperature simultaneously at multiple positions. For most individuals, testis size (measured via ultrasound) and serum testosterone levels were also measured. Young males (2-9 years) displayed uniformly high temperatures along the length of their colons - there was no measurable influence of the reproductive CCHE on colonic temperatures. In older males (10-43 years) colonic temperatures were dependent upon position; temperatures measured at the CCHE were on average 0.5oC (32.9oF), and maximally 1.7oC (35oF), cooler than those measured outside this region. Longitudinal records from individuals that became sexually mature during the course of the study also showed that temperatures at the CCHE decreased as testis size and testosterone levels increased. These results, the first to demonstrate that CCHE function changes with age and reproductive maturity, also illustrate the importance of long-term field studies to enhance our understanding of marine mammal biology.